10 ZOOLOGY 



masses called cells (see Chapter III). These unit-masses of 

 protoplasm are in some degree independent of one another, 

 because normally each tends to form a wall about itself; and 

 yet it is highly probable that the whole protoplasm of an ani- 

 mal is physically continuous by means of delicate connections 

 between the units. The life of the cells is not quite the same 

 thing as the life of the organism to which they belong, for 

 in animals composed of more than one cell a cell may die with- 

 out involving the death of the animal. The protoplasm of the 

 cell may also retain life for a time after separation from the 

 living animal or after the animal as a whole has ceased to live. 

 This is shown by the fact that cells may be taken from the 

 body of young organisms and, if kept nourished under conditions 

 similar to those of the body from which they are taken, will not 

 merely live but will continue to divide and grow almost 

 indefinitely. This has been shown to be true of several classes 

 of cells. 



1 6. Protoplasm. While we describe protoplasm as the 

 "physical basis of life" (Huxley), we no longer think of it as a 

 constant or definite material with an exact composition. It 

 is rather a complex mixture of substances, some of which are 

 themselves very complex compounds. These substances in 

 the mixture are continually bringing about changes in one 

 another. It is inevitable that such a mixture of changeable 

 substances should itself be most unstable and variable. It is 

 agreed that there is much in common in all protoplasm, even 

 in protoplasm as far- apart as that of plants and animals; and 

 yet it is also true that the protoplasms of different animals and 

 of different parts of the same animal are definitely different. 

 This difference is ^hown by the difference in the work they can 

 perform, as in muscles and nerve cells. 



Perhaps this is the most wonderful thing in life : that a sub- 

 stance, so complex and so variable that it is not exactly the 

 same any two moments in succession, should still be so constant 

 that a small amount of it split off generation after generation 

 should transmit all the essential qualities of the species to which 

 it belongs, whether a paramecium or a man or an oak. 



